Conventionally, a technique for providing mobile communication of the entire network composed of a movable terminal group and a router has been examined in IETF NEMO-WG.
NEMO (Network Mobility) described in the document Network Mobility (NEMO) Basic Support Protocol (written by Vijay Devarapalli et al., RFC3963) is a technique that implements data communication in a network (referred to as “mobile network” hereinafter) composed of a movable terminal group, and of a mobile router connecting to an external network such as the Internet, by means of the mobile network and a home agent that manages the position of the mobile network.
FIG. 10 illustrates a conventional mobile network system disclosed in the document.
In FIG. 10, terminal 6 connects to mobile network 3 managed by mobile router (MR) 5 and mobile router (MR) 4. Mobile router 5 connects to home network 2 managed by home agent (HA) 10 and to global network 1 such as the Internet through home gateway (HGW) 8. Mobile router (MR) 4 connects to global network 1 through access router (AR) 9.
A description is made for path control in such a mobile network system using FIG. 11.
FIG. 11 is a sequence diagram illustrating path control in a conventional mobile network system.
In FIG. 11, when the power to mobile router 4, 5 turns on, mobile router 4 connects to access router (AR) 9, and mobile router 5 connects to home network (HNW) 2 (step S1).
It is assumed that mobile router 4 is a mobile phone, and mobile router 5 is a PDA having a wireless LAN interface, for example. In this case, mobile phone (MR4) connects to a base station (AR9) placed out of home even if the user is at home, not directly connecting to home network 2 at home. The PDA (MR5) connects to home network 2 through a wireless LAN interface.
In this way, some mobile routers results in connecting to home network 2, and the others not connecting even in the same home.
That is, mobile router 4 creates a care-of address (referred to as “CoA” hereinafter) to perform position entry in home agent 10 (step S2) in order to connect to an external network other than home network 2. Mobile router 4 informs home agent 10 of the network prefix (referred to as “MNP3” hereinafter) of network 3 simultaneously with the position entry.
Having acquired a path addressed to MNP3 via mobile router 4, home agent 10 starts path advertisement for informing the acquisition (step S3). Mobile router 5 as well has a path addressed to MNP3, and thus advertises the path addressed to MNP3 to home network 2 (step S4). The information on the path addressed to MNP3 advertised by this mobile router 5 is assumed to be metric=1. The metric generally indicates the number of hops to a destination, where a path with a smaller metric is selected more preferentially.
The above information enables home gateway 8 to learn a relay destination to which a packet addressed to MNP3 is to be transferred.
Here, home gateway 8 determines a next hop as a path addressed to MNP3 to be mobile router 5 (step S5). Accordingly, home gateway 8 transmits data addressed to terminal 6 to mobile router 5 (step S10).
Next, when the user moves to a location with an access point for a wireless LAN after going out of home carrying a PDA (MR5), the PDA (MR5) connects to access router 9 for the wireless LAN (step S6). Then, the PDA (MR5) creates a CoA to perform position entry in home agent 10 (step S7).
Receiving a notice from mobile router 5, home agent 10 periodically advertises path information same as that advertised in step S3, to the home network (step S8). However, a packet for advertising a path is generally sent out at time intervals of several tens of seconds to prevent the communication traffic load from increasing. Accordingly, home gateway 8 tries transmitting a packet with mobile router 5 as a relay destination even if home gateway 8 ceases receiving a path advertising packet from mobile router 5 (step s11).
After that, home gateway 8 updates the path table on the basis of path information from home agent 10 and selects home agent 10 as a new relay destination to MNP3 (step S9). Consequently, data destined for terminal 6 is delivered through mobile router 4 via home agent 10 (step S12).
In this way, the user can communicates with a terminal and the like on the Internet from terminal 6 by connecting to the home network through mobile router 5 while at home, and via a mobile phone (MR4) outdoors.
However, in step S5, if home gateway 8 receives a path advertisement packet from home agent 10 (step S3) earlier than that from mobile router 5 (step S4), home gateway 8 results in selecting home agent 10 as a relay destination for a packet addressed to MNP3 because both metrics are ‘1’.
In this case, a packet from terminal 7 addressed to terminal 6 is more efficiently sent from home gateway 8 to terminal 6 via mobile router 5 because a packet merely flows through the home network. However, home gateway 8 results in selecting a redundant path that passes through home agent 10 and mobile router 4.
In step S6, when the user leaves home and his/her carrying PDA (mobile router 5) connects to access router 9, home gateway 8 does not updates a path table for several tens of seconds even if home agent 10 is newly advertising a path. Accordingly, a packet addressed to terminal 6 during this period does not reach terminal 6 but disappears.